He joined the drive to defund President Barack Obama's health care law, though his voice grew softer as the resulting government shutdown and his party sank in polls. He then turned to championing social issues like legislative prayer.

On Saturday, Rubio delivered the keynote address at a fundraiser for the Florida Family Policy Council, an evangelical group that led the successful 2008 effort to ban gay marriage in the state. And next week, the potential presidential candidate plans to deliver what aides described as a major foreign policy speech at the conservative American Enterprise Institute.

Taken together, Republicans say the actions suggest two things: that Rubio is trying to reconnect with activists still smarting over his support for an immigration overhaul that included a path to citizenship for millions of immigrants living here illegally, and that he's trying to find an issue that resonates with conservatives, in the way Texas Sen. Ted Cruz is linked to fighting "Obamacare" and Kentucky Sen. Rand Paul to criticizing the president's use of drone strikes.

That's important if Rubio wants to stand out in a potentially crowded GOP presidential field, where he is generally viewed as less strident than Cruz and former Sen. Rick Santorum but more conservative than New Jersey Gov. Chris Christie. Rubio's advisers say the senator long has emphasized his conservative positions and would benefit from the fact that, unlike others, he's able to talk about them in a way that doesn't turn off voters from other parts of the political spectrum.

"There is still a space in the Republican primary field for someone to emerge as the conservative alternative to Christie," said Scott Reed, a Republican who ran Bob Dole's 1996 campaign. "Rubio's at the front of the line, if he chooses to run."

To do so successfully, Rubio would need to rekindle the conservative fire among the tea party voters who elevated an obscure state legislator into a national sensation -- and who are poised to help christen the next GOP standard-bearer. Right now, Rubio is so closely associated with the stalled immigration bill that at a conference of conservatives this summer he was heckled with cries of "No amnesty!"

These days, he rarely mentions immigration. And after months of arguing for the passage of the comprehensive bill he helped write, Rubio says he now favors the piecemeal approach of House leaders, who have focused primarily on border security and enforcement. He has said he's being "realistic" about the prospects of far-reaching changes in the Republican-dominated lower chamber.

Meanwhile, Rubio has used his perch on the Senate foreign relations and intelligence committees to push for stiffer sanctions on Iran and greater accountability on foreign aid.

He's also emphasizing his right-leaning positions on social issues, which aides say is simply a reflection of the Florida senator's conservative passions; he is a devout Catholic who wears a bracelet highlighting his opposition to abortion rights.

"Sen. Rubio is a committed movement conservative who is active on almost every front in the fight for the values that make America great," said spokesman Alex Conant. "The values and principles he's fighting for right now are the same ones he's been fighting for as long as he's been in public office."

Last month, Rubio won standing ovations from activists at the Values Voter Summit in Washington when he affirmed his Christian faith and denounced what he called a "rising tide of intolerance" toward social conservatives.

"I've also been lectured, as many of you have, about how we need to stop talking about social issues if we want to win elections," Rubio told the crowd. "But if we're serious about saving the American dream, we can't stop talking about these issues. ...The moral well-being of our people is directly linked to their economic well-being."

As the Supreme Court considered a case about the constitutionality of prayer in government meetings, Rubio wrote op-eds for Christian and conservative media outlets defending the role of public religious expression. In interviews, Rubio backed the town of Greece, N.Y., which sought to overturn an appeals court ruling that barred the Rochester suburb from beginning its council meetings with mostly Christian prayers.

This month, he opposed Senate legislation that would prohibit workplace discrimination against gay, bisexual and transgender Americans. In a statement, his office said Rubio was concerned the bill could result in "frivolous lawsuits" for small businesses and wanted to ensure that "religious freedoms under the First Amendment are protected." Last week, he co-sponsored a bill that would ban abortion after 20 weeks of pregnancy.

Some of the actions have drawn criticism.

In recent months, Rubio quietly withdrew his support for the nomination of a Miami judge who would have been the first openly gay black man on the federal bench. Although he initially backed William Thomas for a seat on the U.S. District Court for the Southern District of Florida last year, a spokeswoman said further review raised concerns about the judge's "judicial temperament and his willingness to impose appropriate criminal sentences" in two cases.

Legal and civil rights groups have alleged Rubio's motives are political. They note that in one case, a hit-and-run in which Rubio thought the sentence was too lenient, the lead prosecutor and an administrative judge wrote the senator saying Thomas had acted fairly.

"To me, that would not be a basis to prevent the confirmation process from going forward," said Yolanda Strader, president of Miami's largest association for black lawyers.

While some of Rubio's actions could alienate moderates and independents in a general election, they have not gone unnoticed in the early-voting states that play an outsized role in picking presidential nominees. There, social issues are often at the top of Republicans' concerns.

"Marco Rubio, in looking at his entire body of work, fits very well into the mainstream of the values of South Carolina," said Bob McAlister, a GOP consultant who helped run Sen. John McCain's campaign there in 2008.

"We want a president who is conservative to the core on foundational issues but at the same time realistic and has the ability to get things done," he said.